1. Field of the Invention
The present invention relates to semiconductor circuits, and more particularly to power-up circuits.
2. Description of Related Art
A common goal in circuit design is to ensure that the circuit powers up in the proper state. Power supply lines are routed to different parts of the circuit, and thus possess parasitic capacitance.
Therefore, upon power-up, it takes a certain amount of time for the power supply lines to reach the final supply voltage level. If the circuit is activated before the supply lines reach a high enough voltage level, sensitive circuits, such as latches, may assume an incorrect state, and also failures such as latch-up can occur.
These concerns become even greater with devices in which an externally provided supply voltage is converted to an internal supply voltage. Semiconductor devices offering low power consumption as an option are such devices. For example, many 5V devices offering low power as an option, convert the externally provided 5V supply to an internal supply voltage of 3.3V, which voltage is provided to internal circuits via the internal supply lines.
In such devices, the voltage converter that converts the 5V voltage to 3.3V consumes static power. The amount of power consumed is proportional to how rapidly the voltage on the internal supply lines needs to be raised upon power-up. The faster the rise time, the higher is the static power consumption. High static power consumption can be prohibitive in low power devices.
Therefore, voltage converters in many low power devices consume little static power at the expense of slower internal supply voltage rise time. The internal supply voltage rise time can be especially slow in larger devices wherein the parasitic capacitance of the internal supply lines is large. With such slow rise time, activating the device before the internal circuits have stabilized and assumed proper states can cause the device to power-up in a faulty state. Further, premature activation of circuits can lead to device failures due to latch-up.
Problems related to premature device activation have been addressed by device specifications stating that the device can be enabled only a certain period of time after power is turned on. Usually, the time period is a conservative estimate of the time it takes for the internal supply lines to reach the final voltage level.
However, in many systems using a single power supply, for example in personal computers, implementation of the chip enable timing can be difficult or impossible since the chip enable signal is generated by a circuit which itself needs time to power up before it can reliably provide the chip enable signal.